Your husband's parents. It's on him. Do not allow yourself to listen to their criticisms and complaints any longer. He needs to put on his big boy pants and deal with them and defend you and your efforts. ITA that the cousin relationship is worth your efforts.
What about the parents of the cousins? Husbands siblings?
Hubby needs to man up and your in-laws sound very whiney and self-centered. Talk to him about this issue NOW and ask him exactly how he plans to deal with it on Mothers Day. Specific action.
I am a new MIL (2017). Believe me, I try to ask nothing of DDIL and as little as possible of DS, especially because they are loving and respectful of me and I would like to keep things that way.